Past rulings have "made politicians think there are no boundaries around what they can do." A Wisconsin case may lead to some limits.

One of the most time-honored and criticized traditions in American politics is for the party in power to draw legislative districts in ways that help keep them in power. The U.S. Supreme Court, though, may soon outlaw at least the most blatant partisan gerrymandering.

On Oct. 3, the nation's highest court will hear oral arguments in a case challenging Wisconsin's state legislative districts. Plaintiffs complain that the map unfairly protects Republican lawmakers from partisan competition. A lower court agreed with that argument last November.

"This could be a huge case if the justices strike down Wisconsin's partisan gerrymander," says Joshua Douglas, a law professor at the University of Kentucky. "That will show that there are some limits to partisan gerrymandering."

The U.S. Supreme Court has ruled against racial gerrymandering repeatedly -- but never against partisan gerrymandering. That "has made politicians think there are no boundaries around what they can do," says Michael Li, senior counsel at the Brennan Center for Justice, which filed an amicus brief supporting the plaintiffs in the Wisconsin case.

The last time the Supreme Court heard a partisan gerrymandering case, in 2004's Vieth v. Jubelirer, the justices were divided. Justice Anthony Kennedy wrote that there could be such a thing as an unconstitutional partisan gerrymander -- but only if the courts had a "workable standard" for determining when partisans had crossed that line.

The Wisconsin case, known as Whitford v. Gill, represents an attempt to come up with such a standard.

To prove their argument that partisan gerrymandering in the state exceeded what's constitutional, the plaintiffs used a new political science measurement known as the efficiency gap, which looks at how votes translate into victories. According to this argument, all votes cast for a losing candidate and any votes for the winner beyond what was needed to win are considered “wasted.” If too many districts have lopsided outcomes (where the party that drew the map “wasted” significantly fewer votes), the argument goes, that shows that the party that drew the map sought to game the system, creating districts that are totally safe for one party or the other and diluting its strength in neighboring districts.

Plaintiffs in the Wisconsin case used the efficiency gap to show that Wisconsin's Assembly map -- as measured by results in the 2012 elections -- was roughly three times more inefficient than the average legislature's. Democrats actually won a majority of the overall vote in Wisconsin's legislative contests in 2012 but came away with only 39 of the Assembly's 99 seats.

"Basically, it didn't matter what we did in an election," says Sachin Chheda, a Democratic consultant in Milwaukee who directs the Fair Elections Project, which organized and launched the Whitford lawsuit. "We could get more votes, but there was no path to a majority in the legislature."

That's because regardless of how the total vote breaks out, what matters is winning by district, says Matt Walter, president of the Republican State Leadership Committee, which helps elect GOP state officials nationwide.

"The redistricting process in and of itself is inherently political and was designed as such at the founding," Walter says. "It was designed to haveaccountable elected officials take the actions of adjusting districts based on population growth."

For their part, Wisconsin Republicans have maintained that they didn't draw the maps to punish Democrats. Rather, they note that most Democrats are clustered in Milwaukee or Madison, while Republican voters are spread out more evenly around the state.

Although the issue is almost always divided along partisan lines (depending on which party is in power), there's growing bipartisan support for putting a stop to partisan gerrymandering. A handful of prominent Republicans -- including Ohio Gov. John Kasich, U.S. Sen. John McCain and former California Gov. Arnold Schwarzenegger -- urged the Supreme Court to use the Wisconsin case to establish a standard for measuring partisan gerrymanders.

"The Supreme Court has said before that partisan gerrymandering can be unconstitutional, but basically it doesn't know how to tell when a plan goes too far," says Annabelle Harless, an attorney with the Campaign Legal Center, which is working with plaintiffs in the Wisconsin case. "They could adopt the test plaintiffs propose, they could in theory come up with their own standard, or they could say it's not justiciable [not an issue for courts to decide]."

Harless notes that the plaintiffs in Wisconsin didn't rely exclusively on the efficiency gap. They demonstrated that Wisconsin legislators acted with partisan intent, namely by unearthing emails that showed they were putting Democrats at a disadvantage. They also argued that the fact that Democrats tend to live in the state's major cities wasn't enough to justify the lopsided nature of the Assembly map.

The makeup of the Supreme Court has changed considerably since 2004 -- the last time it took a partisan gerrymandering case -- but the ideological breakdown of justices is expected to remain the same, with four convinced that partisan gerrymanders are out of bounds and four others believing the exact opposite.

On this question, as in many other cases, Justice Kennedy is expected to remain the swing vote.

The option is spreading at a time of heightened fear of foreign interference in U.S. elections. It has been used in a few local elections and will be available to some voters in the 2020 presidential caucuses.